Several eminent scientists think otherwise, that life in the universe
could have a myriad of possible biochemical foundations ranging from life in
ammonia to life in hydrocarbons and silicon. Silicates have a rich chemistry
with a propensity for forming chains, rings, and sheets.

The Earth was formed largely out of the heavier elements, including carbon
and oxygen. Somehow, Hawking observes, "some of these atoms came to be
arranged in the form of molecules of _DNA_
(http://www.dailygalaxy.com/my_weblog/2012/07/-update-new-research-says-2010-nasa-finding-of-arsenic-based-dna-i
nconclusive.html) . One possibility is that the formation of something
like DNA, which could reproduce itself, is extremely unlikely. However, in a
universe with a very large, or infinite, number of stars, one would expect
it to occur in a few stellar systems, but they would be very widely
separated."

Other prominent scientists have warned that we humans may be blinded by
our familiarity with carbon and Earth-like conditions. In other words, what
we’re looking for may not even lie in our version of a “sweet spot”. After
all, even here on Earth, one species “sweet spot” is another species
worst nightmare. In any case, it is not beyond the realm of feasibility that
our first encounter with extraterrestrial life will not be a solely
carbon-based fete.

Alternative biochemists speculate that there are several atoms and
solvents that could potentially spawn life. Because carbon has worked for the
conditions on Earth, we speculate that the same must be true throughout the
universe. In reality, there are many elements that could potentially do the
trick. Even counter-intuitive elements such as arsenic may be capable of
supporting life under the right conditions. Even on Earth some marine algae
incorporate arsenic into complex organic molecules such as arsenosugars and
arsenobetaines.

Several other small life forms use arsenic to generate energy and
facilitate growth. Chlorine and sulfur are also possible elemental replacements for
carbon. Sulfur is capably of forming long-chain molecules like carbon.
Some terrestrial bacteria have already been discovered to survive on sulfur
rather than oxygen, by reducing sulfur to hydrogen sulfide.

Nitrogen and phosphorus could also potentially form biochemical molecules.
Phosphorus is similar to carbon in that it can form long chain molecules
on its own, which would conceivably allow for formation of complex
macromolecules. When combined with nitrogen, it can create quite a wide range of
molecules, including rings.

So what about water? Isn’t at least water essential to life?

Not necessarily. Ammonia, for example, as we mentioned above has many of
the same properties as water. An ammonia or ammonia-water mixture stays
liquid at much colder temperatures than plain water. Such biochemistries may
exist outside the conventional water-based "habitability zone". One example
of such a location would be right here in our own solar system on Saturn's
largest moon Titan.

Hydrogen fluoride methanol, hydrogen sulfide, hydrogen chloride, and
formamide have all been suggested as suitable solvents that could theoretically
support alternative biochemistry. All of these “water replacements” have
pros and cons when considered in our terrestrial environment. What needs to
be considered is that with a radically different environment, comes
radically different reactions. Water and carbon might be the very last things
capable of supporting life in some extreme planetary conditions.

While some of these scenarios may seem the stuff of science fiction, it's
important to keep in mind that the foundations of _life on Earth_
(http://en.wikipedia.org/wiki/Organism) , the association of a protein with a
nucleic acid when view abstractly, does little to convey the endgame wonders such
as blue whales and Mozart's operas.

A billion years from now our descendants may have discovered other systems
of physical life such as plasma within stars which would be based on the
reciprocal influence of patterns of magnetic force and the ordered motion of
charged particles. In fact, such life may well exist within our Sun.

Our personal favorite at The Daily Galaxy is the possibility of life in
Neutron stars which wouldfbe based on the properties of polymeric atoms
which which could form chains that could store and transmit information in a
way that bears an uncanny similarity to the functions of nucleic acids -the
molecules that carry genetic information or form structures within cells.
The image at the top of the page shows the strange blobs found in the
Carina nebula, some of which are seen floating on the upper right, might best
be described as evaporating. Energetic light and winds from nearby stars are
breaking apart the dark dust grains that make the iconic forms opaque.
Ironically the blobs, otherwise known as dark molecular clouds, frequently
create in their midst the very stars that later destroy them. The floating
space mountains pictured above by the orbiting Hubble Space Telescope span a
few light months